The Integrated Services model has an initial deployment scenario of
routers connected together by point to point links. In that situation,
packet scheduling for service classes needs only to be deployed within
the routers, to provide the service overall.

However, many parts of the Internet involve other interconnection
technologies between routers. Two common, but extremely different
situations are:

Routers interconnected via Local Area Networks, such as Ethernet,
FDDI, 100Mbps Ethernet, 100VG, and so forth.

Routers interconnected by so-called ``Non Broadcast Multiple Access''
(NBMA)
networks such as Frame Relay, SMDS and ATM - these are often
referred to as switched clouds, typically because of the way
that they are drawn on inter-network maps.

The Integrated Services over Specific Lower Layers, Working Group
of the IETF has defined the mapping of some of the IP level services
onto services provided at the lower layer.

In some cases, the Link Layer as currently deployed cannot support the
upper layer services with any reasonable guarantee, and so some
enhancement is typically called for - one such case is Ethernet where
the shared Media Access is so non-deterministic, that an enhancement
based on some form of distributed bandwidth manager is required to
provide anything beyond best effort IP service over an Ethernet hop.

In the case of NBMA networks, particularly ATM, a much richer variety
of services is available at the lower layer for IP to utilise. In
fact, there are multiple possible mappings from the high level IP
service, to the proposed ``Bearer Service Classes'' of ATM - for
example:

Guaranteed Service could be provided over CBR or rtVBR.

Controlled Load could be provided over CBR, rtVBR, nrtVBR or ABR

Best Effort could be provided over CBR, rtVBR, nrtVBR, ABR or UBR

Again, this area is very active, and simplification will no doubt be
sought and found by the market.
In the next 3 subsections, we briefly describe the ``pure'' IP approach.